Regular expressions are a powerful way of specifying complex search and replace operations.
The following section is taken (with minor modifications) from the GNU regular expression library documentation, and is Copyright (C) Free Software Foundation.
A regular expression describes a set of strings. The simplest case is one
that describes a particular string; for example, the string `
regarded as a regular expression matches `
foo' and nothing else.
Nontrivial regular expressions use certain special constructs so that they
can match more than one string. For example, the regular expression
foo|bar' matches either the string `
foo' or the string
bar'; the regular expression `
c[ad]*r' matches any of the strings
cadddar' and all other
such strings with any number of `
a''s and `
Regular expressions have a syntax in which a few characters are special
constructs and the rest are ordinary. An ordinary character is a
simple regular expression which matches that character and nothing else. The
special characters are `
]' , `
)' and `
\'. Any other
character appearing in a regular expression is ordinary, unless a `
For example, `
f' is not a special character, so it is ordinary,
and therefore `
f' is a regular expression that matches the string `
and no other string. (It does not match the string `
o' is a regular expression that matches only `
Any two regular expressions a and b can be concatenated. The result is a regular expression which matches a string if a matches some amount of the beginning of that string and b matches the rest of the string.
As a simple example, we can concatenate the regular expressions
f' and `
o' to get the regular expression `
which matches only the string `
fo'. Still trivial.
Note: special characters are treated as ordinary ones if they are in
contexts where their special meanings make no sense. For example,
*foo' treats `
*' as ordinary since there is no preceding
expression on which the `
*' can act. It is poor practice to depend on
this behavior; better to quote the special character anyway, regardless of
where is appears.
The following are the characters and character sequences which have special meaning within regular expressions. Any character not mentioned here is not special; it stands for exactly itself for the purposes of searching and matching.
a.b' which matches any three-character string which begins with `
a' and ends with `
fo*', the `
*' applies to the `
o', so `
fo*' matches `
f' followed by any number of `
The case of zero `
o''s is allowed: `
fo*' does match
*' always applies to the smallest possible preceding
expression. Thus, `
fo*' has a repeating `
o', not a repeating
+' is like `
*' except that at least one match for the preceding pattern is required for `
+'. Thus, `
c[ad]+r' does not match `
cr' but does match anything else that `
c[ad]*r' would match.
?' is like `
*' except that it allows either zero or one match for the preceding pattern. Thus, `
c[ad]?r' matches `
cr' or `
car' or `
cdr', and nothing else.
[ ... ]'
[' begins a character set, which is terminated by a `
]'. In the simplest case, the characters between the two form the set. Thus, `
[ad]' matches either `
a' or `
d', and `
[ad]*' matches any string of `
a''s and `
d''s (including the empty string), from which it follows that `
c[ad]*r' matches `
car', et cetera.
Character ranges can also be included in a character set, by writing two
characters with a `
-' between them. Thus, `
[a-z]' matches any
lower-case letter. Ranges may be intermixed freely with individual
characters, as in `
[a-z$%.]', which matches any lower case letter or
%' or period.
Note that the usual special characters are not special any more inside a
character set. A completely different set of special characters exists
inside character sets: `
-' and `
To include a `
]' in a character set, you must make it
the first character. For example, `
a]' matches `
]' or `
To include a `
-', you must use it in a context where it cannot possibly
indicate a range: that is, as the first character, or immediately
after a range.
[^ ... ]'
[^' begins a complement character set, which matches any character except the ones specified. Thus, `
[^a-z0-9A-Z]' matches all characters except letters and digits.
^' is not special in a character set unless it is the first character.
The character following the `
^' is treated as if it were first (it may
be a `
-' or a `
^foo' matches a `
foo' which occurs at the beginning of a line.
^' but matches only at the end of a line. Thus, `
xx*$' matches a string of one or more `
x''s at the end of a line.
\'), and it introduces additional special constructs.
\' quotes special characters, `
\$' is a regular
expression which matches only `
$', and `
\[' is a regular
expression which matches only `
[', and so on.
For the most part, `
\' followed by any character matches only that
character. However, there are several exceptions: characters which, when
preceded by `
\', are special constructs. Such characters are always
ordinary when encountered on their own.
|' in between form an expression that matches anything that either a or b will match.
foo|bar' matches either `
foo' or `
bar' but no other
|' applies to the largest possible surrounding expressions. Only a
( ... )' grouping can limit the grouping power of
( ... )'
|' alternatives for other operations. Thus, `
(foo|bar)x' matches either `
foox' or `
*' to operate on. Thus, `
ba(na)*' matches `
bananana', etc., with any (zero or more) number of `
This last application is not a consequence of the idea of a parenthetical
grouping; it is a separate feature which happens to be assigned as a second
meaning to the same `
( ... )' construct because there is no
conflict in practice between the two meanings. Here is an explanation of
( ... )' construct, the matcher remembers the beginning and end of the text matched by that construct. Then, later on in the regular expression, you can use `
\' followed by digit to mean ``match the same text matched the digit'th time by the `
( ... )' construct.'' The `
( ... )' constructs are numbered in order of commencement in the regexp.
The strings matching the first nine `
( ... )' constructs appearing
in a regular expression are assigned numbers 1 through 9 in order of their
\1' through `
\9' may be used to refer to the text matched by
the corresponding `
( ... )' construct.
For example, `
(.+)\1' matches any non empty string that is composed of
two identical halves. The `
(.+)' matches the first half, which may be
anything non empty, but the `
\1' that follows must match the same exact
\bfoo\b' matches any occurrence of `
foo' as a separate word. `
\bball(s|)\b' matches `
ball' or `
balls' as a separate word.
Also the replacement string has some special feature when doing a regular
expression search and replace. Exactly as during the search, `
by digit stands for ``the text matched the digit'th time by the
( ... )' construct in the search expression''. Moreover, `
represent the whole string matched by the regular expression. Thus, for
instance, the replace string `
\0\0' has the effect of doubling any string
Another example: if you search for `
(a+)(b+)', replacing with
\2x\1', you will match any string composed by a series of `
followed by a series of `
b''s, and you will replace it with the
string obtained by moving the `
a' in front of the `
x' inbetween. For instance, `
aaaab' will be matched and
replaced by `
Note that the backslash character can escape itself. Thus, in order to put a
backslash in the replacement string, you have to use `